1. Field of the Invention
The invention relates to a device for transferring heat between a panel designed to be heated by solar radiation and a wall, such as for example the wall of a building.
2. Description of the Related Art
There are already known arrangements applied to the face of a building for capturing solar energy and heating a receiving medium such as, for example, an outside wall of the building or a volume of air inside the building.
These arrangements for capturing solar energy are sometimes also called xe2x80x9csolar panelsxe2x80x9d or xe2x80x9ctransparent insulationxe2x80x9d.
An arrangement of this type is described for example in French Patent No. 92 03962.
This known device comprises a number of substantially horizontal shaped elements spaced apart and superposed to form a curtain having an external face designed to be exposed to solar radiation and an internal face designed to be arranged facing the receiving medium which is to be heated. This device thus permits the transfer of the heat while at the same time constituting an efficient insulation.
In such a device there is normally provided a conversion plate which forms part of the panel and which ensures the conversion of the solar energy into heat.
The transfer devices of the solar panel type are generally applied directly to the face of the building to allow not only a transfer of heat derived from the solar radiation collected, but also a thermal insulation when the solar radiation is absent.
The invention takes as its starting point an appreciation of the fact that the solar panels known at present are not able to deal with all the situations which are encountered in practice.
The applicants have come to appreciate that in fact four different situations exist and need to be dealt with:
a) the heat provided by the solar radiation is welcome and it needs to be transferred to the surface, for example a wall, to be heated.
b) an excess of solar heat is present and it is therefore necessary to avoid any transfer of heat between the panel and the surface to be heated.
c) there is no solar heat available and it is necessary it to avoid any loss of heat from the surface, for example the wall, towards the outside.
d) there is no incoming solar heat present but there is an excess of heat present in the surface. It is therefore necessary, to the extent possible, to reject this heat to atmosphere.
The principal aim of the invention is to provide a heat transfer device of the type defined in the introduction which is capable of handling in priority the first three situations (a, b, c).The fourth situation (d: cooling at night) demands in principle a supplementary logic and the handling of this situation is only really needed in certain climates.
The device according to the invention comprises, for this purpose, an insulating plate designed to be mounted between the panel and the wall surface, parallel to these, in such a way as to define a closed external space between the panel and the insulating plate and a closed internal space between the insulating plate and the wall, as well as air circulating means interposed between the external space and the internal space and capable of being put selectively in at least one of the following states:
an open state allowing circulation of air between the external space and the internal space and thus allowing thermal transfer between the panel and the wall;
a closed state preventing circulation of air between the external space and the internal space and thus preventing thermal transfer between the panel and the wall.
Thus, depending on the state (open or closed) of the air circulating means, it is possible to cause or on the contrary to prevent a transfer of heat between the panel and the wall. When the air circulating means are in an open state, a transfer of heat takes place between the panel and the wall. By contrast, when the air circulating means are in the closed state, there is no transfer of heat but on the contrary a good thermal insulation.
The invention thus achieves mastery over the transfer of heat by convection while preventing the direct conductive and/or radiative contact between the panel and the wall by interposing a particular device. This device, with its two empty spaces and its insulating plate between the panel and the wall, guarantees low conduction of heat as well as low transfer by radiation, while allowing the transfer of heat by convection.
The panel to which the invention is applied could be in any position: vertical, inclined or horizontal.
In principle the wall surface to which the heat is to be transferred is a masonry wall, but it could also take the form of a flat roof or even a vessel containing a liquid.
In one preferred embodiment of the invention the air circulating means comprise at least two openings provided in the insulating plate and each designed to provide communication between the external and internal space.
In practice it has been found that a single opening allows only very limited exchange of air.
The term xe2x80x9copeningxe2x80x9d such as is used here is intended to designate generally any passage, possibly including restrictions or bends, providing communication between the internal and external spaces of the device.
Preferably the air circulating means comprise two openings positioned substantially at the same height with respect to one wall of the device such as to allow limitation of the flow of air passing from one opening to another. By contrast, if the openings are not at the same height it is necessary to be able to block the natural flow of air, for example by means of a flap valve.
Preferably a partition or dividing wall is arranged in each of the external space and the internal space to define, in each of the said spaces, at least one path for circulation of generally U shape having two branches, one on each side of the partition.
This arrangement of U-shaped circuits consists essentially of vertical walls. It ensures good scouring of the surfaces and facilitates the circulation of the air in each of the external and internal spaces, as well as a circulation of the air from one to the other. An effect analogous to scouring could be obtained by other means, for example by bends or diversions, or even by other types of circuit for circulating air between the two above-mentioned spaces.
Preferably the said air circulating means comprise at least one first opening providing communication between a first branch of the external space and a first branch of the internal space, and at least one second opening providing communication between a second branch of the external space and a second branch of the internal space.
According to another feature of the invention the air circulating means comprise at least two openings extending through the insulating plate and of which at least one is provided with a flow regulating member designed to adopt one or the other of the open and closed conditions.
In one embodiment of the invention the control member comprises an electric or similar fan designed to be put into operation or out of operation corresponding respectively to the open condition and the closed condition.
Thus, when the fan is in operation a circulation of the air is obtained simultaneously in each of the external and internal spaces and from one space to the other. By contrast, when the fan is out of action the circulation of air is stopped.
In another embodiment the control member comprises an electric fan and a flap valve which is situated in the same opening as the electric fan and/or in another opening.
Preferably use is made of a gravity-operated flap which opens under the pressure of the air and closes under gravity. This valve is closed by gravity when the fan is not providing the pressure necessary to open it. When the fan and the valve are placed in two different openings, these latter are not necessarily at the same height. This valve could even be omitted if the two openings are situated at the same height, preferably high up or low down.
When the device is to used on a flat roof it is preferable to provide, in addition to the fan, a flap valve in each opening.
In place of a gravity-operated valve it is possible also to make use of a flap operated by a bimetal strip heated by the solar radiation or an electro-magnetically controlled flap. In certain versions one could even accept having a flap controlled manually by mechanical means.
When insolation is present and the transfer of heat to the wall surface is closed off, the temperature could attain levels which, for certain materials, in particular insulation, are excessive.
With this in mind, the invention provides means for protection against the occurrence of excessive temperatures.
One measure for this purpose consists in positioning a supplementary plate, which we call a stopper plate, between the panel and the insulating plate in order to create a closed blocking space between the external space and the insulating plate. If necessary this blocking space could be ventilated, preferably by a flow of atmospheric air through a fixed opening low down and another opening with- a valve higher up. This valve could be operated by a bimetallic strip when the temperature reaches a certain predetermined threshold. Should the occasion arise, the excess heat is removed to atmosphere.
Another step consists in placing an intermediate plate, which we call a conversion plate, between the panel and the insulating plate to create a first air circuit (or heating circuit) designed to transfer the heat between the intermediate plate and the wall surface or vice versa, and to create a second air circuit (or cooling circuit) designed to reject the heat to the outside. Equally it is provided that the first air circuit and the second air circuit have respectively first air circulating means and second air circulating means which are capable of being placed selectively and independently of one another in an open condition or a closed condition.
The conversion plate transforms the solar energy into heat and it then replaces that which normally comprises the panel.
The first circuit allows the transfer of the heat from the conversion plate to the wall surface or vice versa, depending on whether one is in a winter or a summer regime. The second circuit is designed to reject the heat to the outside: to evacuate the heat from the wall surface to the outside and/or extract the excess heat from the intermediate plate to the outside.
Preferably the first air circuit is a closed circuit capable of communicating with the outside through a small pressure-equalising opening, whereas the second air circuit is a closed circuit or is open to the outside.
By the expressions xe2x80x9cclosed circuitxe2x80x9d and xe2x80x9copen circuitxe2x80x9d it is intended to make clear that the circulation of the air takes place in one or several spaces or volumes, respectively without communication with the outside and with communication with the outside.
The device of the invention furthermore includes control means designed to control the flow regulating member, for example the fan.
In one embodiment of the invention these control means comprise manual control means.
In another embodiment these control means comprises at least two temperature sensors designed to detect the temperatures respectively of the panel (or of a conversion plate) and of the wall surface, and to control the flow regulating member as a function of the difference in temperature between the panel (or conversion plate) and the wall surface.
In one embodiment which has been developed, these control means comprise electronic means.
Preferably the control means comprises at least one photo-responsive component designed to detect the insolation on a panel and to generate an electrical voltage representative of the insolation.
The photo-responsive component could be a photo-voltaic cell which will allow the generation of energy which can be used directly to drive the fan. However one could also make use of photo-transistors, photo-diodes or photo-resistances which provide a signal which can be used directly (through the intermediary of an amplifier) to operate the fan, or indirectly as data for a calculation by a logic circuit.
By way of example, one could make use of a photo-transistor coupled to a xe2x80x9cDarlingtonxe2x80x9d type of circuit which delivers an output power sufficient to drive a fan operating on a direct voltage of 12 volts.
The electrical voltage thus generated could be used either directly as energy for driving the flow regulating member or in the form of a signal for initiating or halting the operation of the flow regulating member.
In place of a photo-responsive component it is possible to envisage making use of at least one thermo-sensing component, for example a detector of the temperature or of the difference in temperature, a thermostat or a bimetallic strip.
The device according to the invention is preferably in the form of a module of standard or specific dimensions having a frame which supports the components of the device.